1. Field of the Invention
This invention relates to a magnetic recording medium and a novel method of producing a magnetic recording medium and, in particular, to an economically fabricated and improved magnetic recording medium having perpendicular magnetization for high density recording of signals.
2. Description of the Prior Art
In the technology of conventional magnetic recording using tape, sheet or disk recorders in video, audio or other digitized information storage, the signals are recorded on a magnetic recording layer formed on a nonmagnetic base by magnetizing the layer in the plane of the film in a longitudinal direction, which direction is parallel to the direction of relative movement of the medium to the recording transducer or head. This has been referred to as the longitudinal magnetization mode. In a recording process, the information (or bits) to be stored is recorded by changing the state or direction of the magnetization, i.e., a bit of information is created by reversing the magnetization direction of the magnetic medium or creating a flux reversal. When recording in the longitudinal magnetization mode, a strong demagnetization field is created between oppositely magnetized areas on the medium. The existence of this demagnetization field in the longitudinal magnetization recording mode causes attenuation and rotation of the residual magnetization, with the result that an attenuated output is obtained in reproduction of the recorded signal. This attenuation and signal deterioration is accentuated as the linear packing density is increased or the wavelength of recorded signals is decreased in the direction of longitudinal magnetization. The consequence is a limitation on the maximum recording density capable in the longitudinal magnetization mode.
If the magnetic layer can support principally a magnetization normal to the film plane at the remnants, then, if signals are recorded by reversing the magnetization direction in the direction normal to the surface of the magnetic layer following the pattern of the signal (i.e., recording in the so called perpendicular magnetization mode), decreasing the wavelength of the recorded signal, as by increasing the recording density, causes a reduction in the demagnetizing field. Therefore, it will be appreciated that, for an increase in the density of information to be stored in a given area of the magnetic recording medium, recording in the perpendicular magnetization mode (hereinafter referred to as perpendicular recording) is more advantageous than the conventional recording using longitudinal magnetization mode (hereinafter referred to as longitudinal recording). The superiority of using perpendicular recording on magnetic layers having easy magnetization direction normal to the surface of the layer for extremely high density recording has been demonstrated by S. Iwasaki and Y. Nakamura, IEEE Transactions on Magnetics, MAG-13(5), pp. 1272-1277, S. Iwasaki and K. Ouchi, IEEE Transactions on Magnetics, MAG-14(5), pp. 849- 851, September, 1978 S. Iwasaki, IEEE Transactions on Magnetics, MAG-16(1), pp. 71-76, January, 1980 and U.S. Pat. No. 4,210,946.
In order to effectively take full advantage of the perpendicular recording, the recording medium should have direction of easy magnetization perpendicular to the film plane, and the medium should also be able to support the magnetization in the direction perpendicular to the film plane at the remnant. In terms of magnetization curve, the magnetic properties should be such that the intrinsic magnetic hysteresis curve taken in conjunction with the applied magnetic field normal to the film plane should be substantially rectangular. Although various recording media and methods for their fabrication have been proposed for use in conjunction with the perpendicular recording mode, they have either been unable, in practice, to achieve total success in optimized perpendicular anisotropy of the media for perpendicular recording mode or have been too expensive to produce for commercial applications. Examples of perpendicular recording media and fabrication process in the prior art are disclosed in U.S. Pat. Nos. 4,109,287; 4,075,672 and 4,210,946, and in the article of S. Iwasaki et al entitled "An Analysis for the Magnetization Mode for High Density Magnetic Recording", IEEE Transactions on Magnetics, Vol. (MAG-13(5), pp. 1272-1277 (Sept. 1977) and the article of S. Iwasaki entitled "Perpendicular Magnetic Recording" IEEE Transactions on Magnetics, Vol. MAG-16(1), pp. 71-76, (Jan. 1980).
Three principal prior art and methods for fabrication of recording media for perpendicular recording which were described in the previously mentioned patents are summarized respectively as follows.
In U.S. Pat. No. 4,109,287, referred to as Process 1, a magnetic recording film is formed on a substrate wherein the film comprises a multitude of micro pores formed on the surface of aluminum or an aluminum alloy. A magnetic substance is then deposited in each of the micro pores. The process entails a two step electrolytic treatment. The first step involves an electrolytic treatment on an anodic oxide film formed by ordinary anodic oxidation of the surface of aluminum or an aluminum alloy. Columnar vertical pores are grown into the film, each forming an opening in the film surface. By a second electrolytic treatment, a magnetic substance can be made to deposit and pack into the openings of the formed pores as disclosed in this patent.
In U.S. Pat. No. 4,075,672, referred to as Process 2, magnetic powders incorporated in a polymer binder are applied as a coating to a substrate and then subjected to a magnetic field applied in a direction perpendicular to the plane of the substrate before the polymer has cured. Perpendicular orientation of the magnetic easy axis of the powder particles relative to the substrate plane is accomplished by the presence of the applied magnetic field. Final and complete drying of the film is then subsequently accomplished by curing the binder in a dryer.
In U.S. Pat. No. 4,210,946, referred to as Process 3, a magnetic recording layer preferably comprising a chromium-cobalt (Cr-Co) alloy is deposited on a nonmagnetic substrate by sputtering the alloy in a vacuum employing a high frequency electrical source while the substance is usually held at an elevated temperature.
Even though these three methods of producing medium for perpendicular recording have been demonstrated to produce easy direction of magnetization perpendicular to the film plane, each method has several shortcomings which severely limit their applicability for high density perpendicular recording. The shortcoming for each of the above three processes is as follows:
Process 1. Anodic aluminum film. This process requires an aluminum or aluminum alloy surface for creating micro pores by anodic treatment. This process limited in application to supports which must have aluminum on the surface for the anodic treatment. The anodic treatment of aluminum, in general, is very difficult to control in order to create uniformly distributed pores over the treated surface of the aluminum. Also, the pore formation would be affected by the solid (second phase) precipitates and by the grain boundary of the aluminum alloy. The micro pores formed from removal of precipitates during anodic treatment and micro pores formed from the grain boundary are much larger than average thereby contributing to creation of non-uniform magnetic element and cause excess noise during signal recording.
Process 2. Oriented magnetic particles in a polymer binder. The shortcomings of this method are: (A) The orientation of the magnetic particles by applied magnetic field before the drying and final hardening of the binder cannot achieve a high perpendicular anisotropy because of particle relaxation that occurs during hardening. Hence, the resultant medium will not take full advantage of the perpendicular recording phenomena. (B) The dispersion of the magnetic particles in the coating cannot be totally uniform, resulting in a decrease in the signal/noise ratio.
Process 3. Sputtered Co-Cr alloy film. A shortcoming of this process is the necessity to use chromium to dilute the magnetization of Co in order to achieve good perpendicular easy magnetization. The reduction of magnetization by alloying with chromium would reduce the total magnetic flux for reading, and, as a result, reduce the signal/noise ratio for high frequency recording. Also alloying of Co by Cr would drastically reduce the Curie temperature of the media and, particularly, for high Cr content alloy, such as, for example, above 18 atomic percent Cr. The Curie temperature would be reduced to about or below 100.degree. C., making the media unstable for practical applications.
Another shortcoming of this method is the necessity to employ a sputtering method in vacuum limited by a low sputtering rate as specified in the patent. Sputtering in vacuum is a very expensive process and, particularly, the low rate of deposition accompanying the process make it impractical for large scale production of recording medium at a reasonable price.
In order to overcome the above-mentioned problems associated with the existing proposed methods of producing a magnetic recording medium for perpendicular recording, we have discovered an alternative method, exemplifying the electroplating of Co or Co based alloy thin films, to produce magnetic recording media having a highly oriented easy axis of magnetization perpendicular to film plane for perpendicular recording medium and which can be fabricated at an extremely low cost, rendering it highly adaptive to and competitive for a multitude of perpendicular recording applications.
Electroplating processing of Co alloy based thin film has been previously employed as a means for fabricating magnetic recording media. However, such media has been all for the purpose of in-plane recording or longitudinal recording mode. Examples of such processing are found in the article of I. S. Sallo and J. M. Carr entitled "Studies of High Coercivity Cobalt-Phosphorous Electrodeposits", Journal of Applied Physics, Vol 33, No. 3, pp. 1316-1317 (March, 1961) and articles cited therein.
The study of the plating parameters on the preferred orientation of the crystallites or particles in the electroplating process has been previously analyzed for Co based films, such as, the articles of L. Cadorna and P. Cavallotti entitled "Cobalt Plating from Sulfamate Baths", Proceedings of the Symposium on Sulfamic Acid, pp. 253-263, (Milan, May 25-27, 1966); the article of L. Makahara and L. Mahajan entitled "The Influence of Solution pH on Microstructure of Electrodeposited Cobalt", Journal of the Electrochemical Society, Vol. 127, No. 2, pp. 283-288 (February, 1980) and the article of J. Goddard and J. G. Wright entitled "The Effect of Solution pH and Applied Magnetic Field of the Electrodeposition of Thin Single Crystal Films of Cobalt", British Journal of Applied Physics, Vol. 15, pp. 807-814 (1964). These publications deal with parameter effects in electroplating toward obtaining preferred orientation but never achieved conditions to obtain perfect anisotropy desired for perpendicular recording media nor even mentioned or touched on applications or parameters for magnetic layers having perfect perpendicular anisotropy useful and desirable for perpendicular recording.
In addition, various kinds of post treatment to Co or Co alloy based thin films fabricated by the method of this invention can be applied to improve the magnetization curves by providing large integranular separation between the perpendicular particles and further enhance the perpendicular magnetization hysteretic properties of the medium. The post treatment processes are the subject matter and basis of patent applications Ser. No. (D/81004) and Serial No. (D/81089), filed concurrently herewith.